I have 4 units and all of them at the same exact

I have 4 units and all of them at the same exact time without any warning like hissing..the red light turned on. When I removed the inside part it looked like paper had caught on fire. In fact its like soot all around the unit, little pieces. I cleaned all of them and in two weeks the exact same thing has happen. Black soot stuff. My home is smoke free, no pets and for the most part only a sliding door open. I'm really afraid of these things catching on fire now and the fact all four at the exact time and totally different rooms to boot...What do you think the problem is or where do I take these units to be checked. I'm in Sacramento, California